This invention concerns conveyors of the type in which pallets are frictionally driven in an advancing direction by one or more loops of recirculating drive chain, and in which the pallets are returned around a sprocket at either end of the conveyor and are carried back to the beginning of the conveyor on the return segment of the drive chain extending beneath the upper segment of the chain.
Such conveyors are described in U.S. Pat. Nos. 4,681,212; 4,598,818; 4,062,444; 4,793,261; 4,776,453; 4,088,220; and 4,896,763.
In these prior designs the typical arrangement as shown in U.S. Pat. Nos. 4,088,220 and 4,598,818 has involved the use of a triple roller strand chain in which two roller strands are supported from above and below on guide bars, with the third roller strand projecting inwardly and on which the pallets are supported.
This arrangement results in a cantilever loading of the chain pins interconnecting each of the chain roller strands, resulting in bending forces being exerted on the chain pins, which cantilever loading can result in bending of the chain pins as when shock loading occurs.
The guide bar arrangement is relatively complex since the chain must be supported both from above and below, increasing the cost of construction of such conveyors.
The cantilever loading also can result in increased frictional loads exerted between the pallet and inside chain strand which requires greater horsepower to drive the chain, as in such conveyors, the accumulating feature is provided by the slippage of the chain beneath the pallets whenever the lead pallet is stopped.
U.S. Pat. No. 4,062,444 describes an arrangement for center loading the chain, but only in the advancing direction.
In such prior over/under pallet chain conveyors, the pallet must be driven at the end points of the conveyor as by sprocket drives such as to be able to be returned on the lower return segment of the conveyor chain. In many of these prior designs, as in U.S. Pat. Nos. 4,088,220 and 4,598,818, wedging to the chain is used to establish a positive drive at the sprocket.
These arrangements for driving the pallets around the sprockets have limited the sizing of the pallets such that conveyor designs for large parts have not been practical.
U.S. Pat. No. 4,793,261 describes a separate pallet drive disc which engages elements on the pallet, but lower track elements are required to guide the pallets around the disc axis and the pallets are mounted on wheels and driven by chain dogs.
Accordingly, it is an object of the present invention to provide an over/under chain pallet conveyor of the type described in which the loading of the chain is symmetrical in both the advancing and return directions to eliminate the cantilever loading of the chain pins and to decrease the frictional loads created by driving of the pallets by the chain during accumulation, as well as to simplify the construction of the conveyor.
It is another object of the present invention to provide such over/under accumulating pallet chain conveyor in which an arrangement for positive driving of the pallets accommodates a linking of pallets together to be able to accommodate larger sized workpieces and does not require guide tracks for the pallets.